I agree, to dump all this in conversation would be pretty awkward and perhaps outlandish (it’s hard to get a five-point argument laid out in casual conversation sometimes!). I also think you are on the right track by approaching it by asking a question. However, perhaps you could, having the information presented above in mind, ask a clarification question. For instance, you could ask “what do you mean when you say that the unborn is the mother’s body?” After all, the claim is that the abortion is justifiable in virtue of the unborn merely being a part of the mother’s body. So I think the first step in conversation would be to clarify that they are actually intending to make that argument (as opposed to merely repeating a catch-phrase they’ve heard).

After you get clarification, and assuming they maintain a similar stance to the one opposed by the video, you could take it one of two ways: (1) follow up with the question you proposed, “why would you say something like that?” Or (2) counter their position with the information provided by science.

If you take option (1), I think all you will be doing is demonstrating the person’s own ignorance of the statement they just made. But it if they do provide a reason (or reasons) then it will give you a specific direction to go based on what they say. I think I would prefer the second option however.

If you take option (2), I would be careful not to make a statement out of the gate, but rather engage their position with questions. For instance, after asking for clarification, I would have a few different options on what to say:

“Hmm, the thing I don’t understand about that position is that, scientifically speaking, the unborn has a different genetic signature than the mother. How can the unborn be part of the mother’s body and have different DNA?”

“That is an interesting position, but how can the unborn only be part of the mother’s body if it can also be a different gender than the mother?”

“I seem to have a problem with what you’re saying. Maybe you could help? You see, a surrogate mother carries the unborn for the biological mother, but how can this be if the unborn is merely part of the biological mother?”

All of these options are easy to incorporate into conversation in a casual, engaging way. Hope this helps.